public async Task AddCity_ShouldNotAdd_WhenCountryIsInvalid()
{
// Arrange
var responseMessage = "Invalid country selected.";
var cityForCreationDto = new CityForCreationDto()
{
CountryId = 1,
Name = "Belgrade"
};
_unitOfWorkMock.Setup(x => x.Countries.GetById(It.IsAny <int>()))
.ReturnsAsync(() => null);
// Act
var result = await _sut.AddCity(cityForCreationDto);
// Assert
Assert.Equal(responseMessage, result.Value);
}